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MORE ON VISIBILITY - POTENTIAL APPLICATIONS TO PERSONAL INJURY

From time-to-time in personal injury cases, the question arises about whether a hazard was visible to an individual or not. The result is often that someone will trip or slip on the hazard and injure himself or herself.

Considerable studies have been performed over the years on human visual response. These studies generally correlate lighting, object contrast, object colour, and object size to visibility. One such example involves the field of vision.

Normal human vision can be typically described as a cone of approximately 60 degrees radius emanating from the eyes. Objects that appear in the outside edge of this cone typically fall into our peripheral vision. Objects that appear directly in front of our eyes appear in our central cone of vision. Objects in our peripheral vision will appear as fuzzy or incomplete silhouettes. This is a result of the diversity of the photoreceptor cells on the retina (rods and cones). The density is greatest near the fovea, located almost directly behind the pupil. The further from the fovea, the less the density. As a result, the optimum field of vision occurs in a cone of approximately 15 degrees emanating from the eyes. This is where clear seeing will occur.

Here is a simple experiment which can be performed at your desk to measure your peripheral and central cone of vision. Take a pad of large "Post-It" notes and write your name on it in large letters. Next, fit your eyes on an object on the opposite wall in your office. Place the "Post-It" notes at the edge of your desk at your chest. Slowly, slide the notes away from your chest. Be sure to keep your eyes fixed on the object on the opposite wall. Notice how far from your chest the notes become first visible. Notice how far from your chest until you can read your name.

This simple experiment shows the importance of clearly marking, eliminating or reducing tripping hazards that may occur underfoot while individuals have their eyes fixed on an object ahead.

Why would an engineer know this? In designing operator work stations, it is important to place all information that will be significant to the operator, such as alarms, gauges, readouts, etc., in his or her central core of vision in order that it can be easily read while the operator is seated (or standing) in his or her operating position.

 
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